Aminoacyl-tRNA synthetases, which catalyze the aminoacylation of tRNA molecules, are ancient proteins that are essential for decoding genetic information during the process of translation. In higher eukaryotes, nine aminoacyl-tRNA synthetases associate with at least three other polypeptides to form a supramolecular multienzyme complex (Mirande et al., Eur. J. Biochem. 147:281-89 (1985)). Each of the eukaryotic tRNA synthetases consists of a core enzyme, which is closely related to the prokaryotic counterpart of the tRNA synthetase, and an additional domain that is appended to the amino-terminal or carboxyl-terminal end of the core enzyme (Mirande, Prog. Nucleic Acid Res. Mol. Biol. 40:95-142 (1991)).
In most cases, the appended domains appear to contribute to the assembly of the multienzyme complex. However, the presence of an extra domain is not strictly correlated with the association of a synthetase into the multienzyme complex.
Mammalian TrpRS molecules have an amino-terminal appended domain. In normal human cells, two forms of TrpRS can be detected: a major form consisting of the full-length molecule (amino acid residues 1-471 of SEQ ID NO:1) and a minor truncated form (“mini TrpRS”; amino acid residues 1-424 of SEQ ID NO:3). The minor form is generated by the deletion of the amino-terminal domain through alternative splicing of the pre-mRNA (Tolstrup et al., J. Biol. Chem. 270:397-403 (1995)). The amino-terminus of mini TrpRS has been determined to be the methionine residue at position 48 of the full-length TrpRS molecule (i.e., residue 48 of SEQ ID NO: 1). Alternatively, truncated TrpRS can be generated by proteolysis (Lemaire et al., Eur. J. Biochem. 51:237-52 (1975)). For example, bovine TrpRS is highly expressed in the pancreas and is secreted into the pancreatic juice (Kisselev, Biochimie 75:1027-39 (1993)), thus resulting in the production of a truncated TrpRS molecule. These results suggest that truncated TrpRS can have a function other than the aminoacylation of tRNA (id.)
Angiogenesis, or the proliferation of new capillaries from pre-existing blood vessels, is a fundamental process necessary for embryonic development, subsequent growth, and tissue repair. Angiogenesis is a prerequisite for the development and differentiation of the vascular tree, as well as for a wide variety of fundamental physiological processes including embryogenesis, somatic growth, tissue and organ repair and regeneration, cyclical growth of the corpus luteum and endometrium, and development and differentiation of the nervous system. In the female reproductive system, angiogenesis occurs in the follicle during its development, in the corpus luteum following ovulation and in the placenta to establish and maintain pregnancy. Angiogenesis additionally occurs as part of the body's repair processes, e.g. in the healing of wounds and fractures. Angiogenesis is also a factor in tumor growth, since a tumor must continuously stimulate growth of new capillary blood vessels in order to grow. Angiogenesis is an essential part of the growth of human solid cancer, and abnormal angiogenesis is associated with other diseases such as rheumatoid arthritis, psoriasis, and diabetic retinopathy (Folkman, J. and Klagsbrun, M., Science 235:442-447 (1987)).
Several factors are involved in angiogenesis. Both acidic and basic fibroblast growth factor molecules that are mitogens for endothelial cells and other cell types. Angiotropin and angiogenin can induce angiogenesis, although their functions are unclear (Folkman, J., Cancer Medicine, pp. 153-170, Lea and Febiger Press (1993)). A highly selective mitogen for vascular endothelial cells is vascular endothelial growth factor or VEGF (Ferrara, N., et al., Endocr. Rev. 13:19-32, (1992)).
The vast majority of diseases that cause catastrophic loss of vision do so as a result of ocular neovascularization; age related macular degeneration (ARMD) affects 12-15 million American over the age of 65 and causes visual loss in 10-15% of them as a direct effect of choroidal (sub-retinal) neovascularization. The leading cause of visual loss for Americans under the age of 65 is diabetes; 16 million individuals in the United States are diabetic and 40,000 per year suffer from ocular complications of the disease, often a result of retinal neovascularization. While laser photocoagulation has been effective in preventing severe visual loss in subgroups of high risk diabetic patients, the overall 10-year incidence of retinopathy remains substantially unchanged. For patients with choroidal neovascularization due to ARMD or inflammatory eye disease such as ocular histoplasmosis, photocoagulation, with few exceptions, is ineffective in preventing visual loss. While recently developed, non-destructive photodynamic therapies hold promise for temporarily reducing individual loss in patients with previously untreatable choroidal neovascularization, only 61.4% of patients treated every 3-4 months had improved or stabilized vision compared to 45.9% of the placebo-treated group.
In the normal adult, angiogenesis is tightly regulated, and is limited to wound healing, pregnancy and uterine cycling. Angiogenesis is turned on by specific angiogenic molecules such as basic and acidic fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), angiogenin, transforming growth factor (TGF), tumor necrosis factor-α (TNF-α) and platelet derived growth factor (PDGF). Angiogenesis can be suppressed by inhibitory molecules such as interferon-α, thrombospondin-1, angiostatin and endostatin. It is the balance of these naturally occurring stimulators and inhibitors that controls the normally quiescent capillary vasculature. When this balance is upset, as in certain disease states, capillary endothelial cells are induced to proliferate, migrate and ultimately differentiate.
Angiogenesis plays a central role in a variety of disease including cancer and ocular neovascularization. Sustained growth and metastasis of a variety of tumors has also been shown to be dependent on the growth of new host blood vessels into the tumor in response to tumor derived angiogenic factors. Proliferation of new blood vessels in response to a variety of stimuli occurs as the dominant finding in the majority of eye disease and that blind including proliferative diabetic retinopathy (PDR), ARMD, rubeotic glaucoma, interstitial keratitis and retinopathy of prematurity. In these diseases, tissue damage can stimulate release of angiogenic factors resulting in capillary proliferation. VEGF plays a dominant role in iris neovascularization and neovascular retinopathies. While reports clearly show a correlation between intraocular VEGF levels and ischemic retinopathic ocular neovascularization, FGF likely plays a role. Basic and acidic FGF are known to be present in the normal adult retina, even though detectable levels are not consistently correlated with neovascularization. This can be largely due to the fact that FGF binds very tightly to charged components of the extracellular matrix and cannot be readily available in a freely diffusible form that would be detected by standard assays of intraocular fluids.
A final common pathway in the angiogenic response involves integrin-mediated information exchange between a proliferating vascular endothelial cell and the extracellular matrix. This class of adhesion receptors, called integrins, are expressed as heterodimers having an α and β subunit on all cells. One such integrin, αvβ3, is the most promiscuous member of this family and allows endothelial cells to interact with a wide variety of extracellular matrix components. Peptide and antibody antagonists of this integrin inhibit angiogenesis by selectively inducing apoptosis of the proliferating vascular endothelial cells. Two cytokine-dependent pathways of angiogenesis exist and can be defined by their dependency on distinct vascular cell integrins, αvβ3 and αvβ5. Specifically, basic FGF- and VEGF-induced angiogenesis depend on integrin αvβ3 and αvβ5 respectively, since antibody antagonists of each integrin selectively block one of these angiogenic pathways in the rabbit corneal and chick chorioallantoic membrane (CAM) models. Peptide antagonists that block all αv integrins inhibit FGF- and VEGF-stimulated angiogenesis. While normal human ocular blood vessels do not display either integrin, αvβ3 and αvβ5 integrins are selectively displayed on blood vessels in tissues from patients with active neovascular eye disease. While only αvβ3 was consistently observed in tissue from patients with ARMD, αvβ3 and αvβ5 both were present in tissues from patients with PDR. Systemically administered peptide antagonists of integrins blocked new blood vessel formation in a mouse model of retinal vasculogenesis.
Hence, anti-angiogenic agents have a role in treating retinal degeneration to prevent the damaging effects of these trophic and growth factors. Angiogenic agents also have a role in promoting desirable vascularization to retard retinal degeneration by enhancing blood flow to cells.